TBR OChem Ch4 Q 76 ---HNMR Signal

[jjunior]

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Hi,

the question is asking for number of singlets shown in HNMR. The answer choices are two, three, six or nine. the correct answer is three as shown below.
but wouldn't the H (two carbon to the left of the carbonyl) also be a singlet? and thus the correct answer is four?

thanks! 😍

 

[The Kraken]

Hi,

the question is asking for number of singlets shown in HNMR. The answer choices are two, three, six or nine. the correct answer is three as shown below.
but wouldn't the H (two carbon to the left of the carbonyl) also be a singlet? and thus the correct answer is four?

thanks! 😍

The carbon #2 2 hydrogens to the left of the carbonyl would be a triplet, just keep going along the ring in the same direction and you see it has two neighboring hydrogens.

 

[jjunior]

i still dont get it 🙁
shouldnt it be a singlet? since the carbon that the H is bonded to, bonds to 3 other Carbons, and thus no neighboring protons to be coupled. am i missing something?

 

[The Kraken]

i still dont get it 🙁
shouldnt it be a singlet? since the carbon that the H is bonded to, bonds to 3 other Carbons, and thus no neighboring protons to be coupled. am i missing something?

You're missing something. The neighboring carbons with hydrogens determine splitting. Go back and review it.

 

[Jepstein30]

Hi,

the question is asking for number of singlets shown in HNMR. The answer choices are two, three, six or nine. the correct answer is three as shown below.
but wouldn't the H (two carbon to the left of the carbonyl) also be a singlet? and thus the correct answer is four?

thanks! 😍

I would answer two to this question.

Either the two CH3 groups are equivalent so no splitting BUT they would only lead to ONE singlet (overlapping signals)

Or the two CH3 groups are not equivalent so there would be two peaks but they would be quartets.

I can see the argument being either (second one is better) because the chemical environments they are in ARE different.

 

[jjunior]

You're missing something. The neighboring carbons with hydrogens determine splitting. Go back and review it.

i think i get it now, but for that particular proton, it would be a quintet(5) instead of triplet correct?

 

[The Kraken]

i think i get it now, but for that particular proton, it would be a quintet(5) instead of triplet correct?

We must be talking about different carbons.

 

[enervate]

I hated this question. How is anyone supposed to know that the two methyl substituents on the carbon bridge can't be rotated to become equivalent?

 

[jmbusclas]

I hated this question. How is anyone supposed to know that the two methyl substituents on the carbon bridge can't be rotated to become equivalent?

Uhh...by looking at the picture?

 

[frochocinco]

aright mr. arrogant, why dont you explain why they are not equivalent, bc they are located on the same sp3 hybridized carbon

 

[jmbusclas]

aright mr. arrogant, why dont you explain why they are not equivalent, bc they are located on the same sp3 hybridized carbon

Indeed, they are on the same sp3 carbon. However, that carbon is unique as it is locked into its conformation because it is part of the bridge over the top of the ring. It is unable to rotate. There is no way for the methyl group pointing to the left to rotate to the right without breaking one of the bonds in the bridge.

The methyl group on the right is closer to the electronegative oxygen and will therefore experience greater deshielding than the methyl on the right...pulling the signal just slightly down field and making it non-equivalent.

 

[enervate]

Thanks jmbusclas. We never analyzed the NMR spectra of non-linear molecules in my organic courses and, to my recollection, TBR never reviewed them. Are there any molecules that could rotate to become equivalent if they were substituents on that bridged carbon?

 

[enervate]

Are there any molecules that could rotate to become equivalent if they were substituents on that bridged carbon?

Bump.